Electrostatic print head and printing station



T. E. BYRD 3,460,156

ELECTROSTATIC PRINT HEAD AND PRINTING STATION Aug. 5, 1969 Filed Dec. 31, 1964 NEGATIVE INVENTOR.

THEODORE E. m0 /%4 W ATTORNEY United States Patent 3,460,156 ELECTROSTATIC PRINT HEAD AND PRINTING STATION Theodore E. Byrd, Phoenix, Ariz., assignor to Borroughs Corporation, Detroit, Mich, a corporation of Michigan Filed Dec. 31, 1964, Ser. No. 422,614

Int. Cl. G01d 15/06 U.S. Cl. 346--74 9 Claims ABSTRACT OF THE DISCLOSURE An electrostatic printing station for controlling the size of latent image charged spots deposited on a charged retentive medium. The printing station is comprised of a print head having a plurality of ionization initiating pin electrodes terminating in a planar face and having an electrically biased flat grid positioned parallel to and closely spaced from said planar pin face, an electrically biased planar back electrode positioned parallel to said grid and spaced away from it to define a gap through which said charge retentive medium is passed and a means to energize a selected pin electrode or selected pin electrodes simultaneously as desired. The size of the charged spot is a function of the spreading of the ionized particles in the area between the grid and the planar pin face.

This invention relates to an apparatus for depositing charged areas upon a record medium and, more particularly, to an electrostatic print station incorporating a print head for depositing relatively large charged spots upon a charge-retentive medium.

Deposition of charged spots has been accomplished by prior art apparatus of the type having a pin electrode facing a planar back electrode or anvil. A record medium is placed upon the back electrode and a high-intensity electric field is established between the pin electrode and the back electrode sufficient to introduce, through field emission, charged particles into the gap existing between the two electrodes. Ions generated by the emission cumulate, and are propagated toward the back electrode by the electric field between the electrodes, causing a charged area to be established upon the record medium. Further details of the electrostatic process and of the means for practicing it are disclosed in U.S. Patent No. 2,955,894, issued Oct. 11, 1960, to Herman Epstein for Page Printing Apparatus and in U.S. Patent No. 2,919,171, issued Dec. 29, 1959, to Herman Epstein and Robert J. Phelps for Page Printing Apparatus, both of which patents are assigned to the assignee of the present invention. The disclosures of these patents are incorporated in the description of this invention.

When a pin electrode is used in combination with the planar electrode, the configuration of the ion stream between the electrodes is conical, with the base of the cone facing the planar electrode in the case where negative voltage is applied to the pin. As a result of this spreading effect, the size of the charged area created upon the record medium Will be a function of the distance between the record medium and the pin electrode; to increase the charged area, the record medium-to-pin electrode space must be increased. An increased gap between electrodes, in turn, necessitates a larger operating voltage.

The size of the charged spot is a function of the electric field intensity in the gap between the electrodes at the time of the commencement of ionization. Because of the random variation of the voltage at which breakdown occurs across the relatively large gap needed to create large spots, the size of the charged area has not heretofore been entirely predictable.

Accordingly, it is an object of this invention to provide an electrostatic printing station for creating upon a record medium large, charged spots of predictable magnitude.

It is another object of this invention to provide an improved print head for producing on a record medium charged spots whose size is substantially independent of print head to printing medium distance.

It is a further object of this invention to provide a printing station wherein the voltage gradient across the record medium is substantially reduced.

These and other objects are achieved by a printing station comprising a print head of the type having a plurality of pin electrodes terminating in a print face. A flat metallic grid, preferably a fine mesh screen, is positioned parallel to and closely spaced from the print face. A planar back electrode or anvil is positioned parallel to the grid and far enough from it to admit a record medium. The grid is negatively biased with respect to the anvil to establish a nondivergent unidirectional field between grid and anvil.

A pulse negative with respect to the grid is applied to selected ones of the pin electrodes. A spark discharge is created between an energized pin and the screen, creating a cloud of positive and negative ions. Negative ions are propelled from within the pin-to-screen gap, through the screen-to-anvil gap, until they impinge upon .a defined area of the record medium. Ions spread during their travel from the pin to the screen, but spread little in the screen-toanvil gap.

While some of the objects and a brief description of the invention have been given above, the invention and its objects will be best understood by referring to the following detailed description and to the accompanying drawings wherein:

FIG. 1 is an isometric view, partly broken way, of a preferred embodiment of a print station incorporating the improved print head.

FIG. 2 combines a cross section of FIG. 1 taken along the lines 22 with a showing, in addition, of a schematic representation of a circuit which may be used to operate the print station.

Referring to FIG. 1, there is shown a print head 11 of insulating material having embedded therein an array of pin electrodes 13. While, for purposes of facilitating drafting, an illustrative 4 x 5 matrix of pins has been shown, it will be understood that the matrix may comprise any number of pins in the array desirable for printing indicia and a 5 X 7 pin array has been used successfully in configurations for printing alphabetic and numerical characters. The pin electrodes 13 terminate in a substantially planar print face 15. Closely spaced by insulator 17 and forming part of the print head 11 is a screen or grid electrode 19 which is parallel to and coextensive with the print face of the print head. A planar back electrode 21 is located in a spaced-apart, sub stantially parallel relationship with the grid electrode 19. Record medium 23 is located between grid electrode 19 and back electrode 21, and in substantial contact with the latter. The combination of print head 11 and back electrode 21 constitutes a print station.

FIG. 2 illustrates the arrangement in more detail. A switching tube 25 and a transformer 27, having a primary winding 29 and a secondary winding 31, are provided for each pin electrode 13 to initiate a discharge between it and the grid electrode 19. Each pin electrode 13 is connected through a current-limiting resistor 33 to a terminal of the secondary winding 31 of its associated transformer 27. The other terminal of each of the secondary windings 31 is connected to negative bias voltage source 35. Each primary winding 29 is connected between a source of positive bias and the plate of a switching tube 25. Each 3 switching tube 25 is controlled through its grid by an individual output of pulse control unit 37. The output V of negative bias voltage source 35 is also connected through line 39 to the screen electrode 19.

In operation, the switching tubes 25 are normally nonconducting. Consequently, the pins 13 and the screen electrode 19 are maintained at voltage V a negative voltage when large spots are desired. The back electrode 21 may be maintained at ground potential. To energize a given pin electrode, its associated switching tube 25 is rendered conductive, causing a large negative voltage pulse to be transmitted to that pin through the coupling action of its associated transformer 27.

Because of this negative pulse, a large potential difference is established between screen electrode 19 and the energized pin electrode 13, causing a spark discharge to occur therebetween. This spark discharge generates both positive and negative ions between the energized pin and the screen electrode.

Spreading of ions occurs in their travel from the pin to the screen. This spreading provides an area of charges at the screen exceeding that at the face of the pulsed pin, and is believed to be due to the divergence of the field between the pin, relatively a point source, and the screen, and to the travel of ions along such diverging field lines.

The screen electrode 19 and the back electrode 21 are both of a planar configuration, situated substantially parallel to each other. Accordingly, since the back electrode is at ground potential while the screen electrode is at some negative V a unidirectional, nondivergent field exists between these two electrodes. Under the urging of this field, negative ions are propelled from the screen through the screen-to-anvil gap, onto the record medium 23 to deposit thereon a charged area of negative polarity. The size of the charged area will approximately equal the area occupied by the cloud of ions in the vicinity of the screen.

Because there is substantially no spreading of ionized particles during their transit from the screen towards the back electrode or anvil, the size D of the charged area upon the record medium is substantially independent of the magnitude of the screen-to-anvil gap. The size of the charged area is determined by the extent of spreading of ionized particles in the area between the screen and the energized pin which is determined primarily by the magnitude of the pin-to-screen gap.

While not to be construed as, limiting the invention to these values, the following values of parameters are those used in a model which was built to incorporate the features shown in FIG. 2:

Pin-to-screen gap A 2 mils. Screen-to-paper gap B 60 mils. Pin diameter C 20 mils.

Screen size 250 lines per inch; 3.35 mil hole width; .65 mil Wire Pin pulse width 100 X sec. Series pin resistor 33 10,000 ohms. Spot size D 95 mils.

It will be noted that a spot size D approximately five times that of pin diameter C has been achieved with a pinto-screen gap A of only 2 mils. The use of a 2-mil gap permits operation with a relatively low pin pulse voltage V which would be impossible if ion spreading were to be accomplished by increasing the pin-to-paper distance B of prior art print heads. Satisfactory operation at relatively low pin pulses voltages can be maintained for space gaps of less than 10 mils.

Experiments with the above model indicate that screento-paper gap B may be increased up to inch without significant effect upon spot size D.

The print station has been described as operating with screen and pins biasted negatively with respect to the back electrode, and with a pin pulse voltage which is negative with respect to the pin and screen bias. This mode of operation is preferred because it makes the pins rather than the screen the cathode, thus providing essentially point sources of electrons.

However, other combinations of operating voltages may be used. The screen and pins may be biased positively with respect to the back electrode, in which event positively charged ions will be attracted onto the recording medium. The pins may be either positively or negatively pulsed with respect to their bias potential, be that potential positive or negative with respect to the back electrode.

Other parameters may be varied, depending upon the size of charged area desired. Screen sizes ranging between 80 lines per inch to 500 lines per inch have been used with a corresponding variation in charged spot size ranging be tween 95 mils and 110 mils. Satisfactory results are available from screen sizes ranging substantially between 50 lines per inch to 1,000 lines per inch. The screens employed in the above model were all electro-formed nickel mesh manufactured by the Buckbee Mears Company, St. Paul 1, Minn. Molybdenum screens could also be used.

Pulse width may also be varied to alter spot size. Thus, with an SO-lines-per-inch screen, increasing the pulse length from 100 microseconds to 200 microseconds resulted in a corresponding increase in charged spot size from 120 mils to 130 mils. Spot size may also be varied by modifying the size of pin ends in the print face. A print head may have different sized pins, larger pin ends being provided for generation of larger spots.

Having described several embodiments of a printing station utilizing an improved print head operable at reduced voltages for creating upon a record medium large charged spots whose size is predictable, and substantially independent of the print head-to-record medium distance in accordance with the invention, it is believed obvious that many modifications and variations of the present invention are possible in the light of the above teachings.

What is claimed is:

1. An electrostatic printing station for establishing an electrically charged area on the surface of a record medium, comprising:

a pair of substantially planar electrodes located to define a space for receiving said record medium substantially in contact with one of said electrodes, said other electrode being perforate,

at least one pin shaped initiating electrode closely spaced from said perforate electrode, located outside said space and substantially normal to said perforate electrode,

means to establish a substantially non-divergent field between said pair of electrodes of insufiicient magnitude to create a discharge, and

means to create a short duration electric field between said initiating electrode and said perforate electrode of sufficient magnitude to create a disruptive discharge between said latter electrodes.

60 2. An electrostatic printing station for depositing charged spots upon a record medium, comprising:

a set of pin electrodes terminating in a substantially planar print face,

a back electrode positioned to support said record medium in spaced apart, substantially parallel relationship with said print face,

a grid electrode substantially coextensive with closely spaced from, and substantially parallel to said print face, located between said print face and said record medium,

a pulse generator for creating a disruptive discharge between said pin electrodes and said grid for generation of clouds of ions, said discharge accomplished by momentarily driving said pin electrodes negative with respect to said grid electrode, and

means to create an electrical field between said grid electrode and said back electrode for propagation of some of said ions onto said record medium.

3. The printing station of claim 2 wherein said means to create an electrical field is a biasing source poled to maintain said grid electrode positive relative to said back electrode.

4. An electrostatic printing station for depositing charged spots upon a record medium, comprising:

a set of pin electrodes terminating in a substantially planar print face,

a back electrode positioned to support said record medium in spaced apart, substantially parallel relationship with said print face,

a grid electrode substantially coextensive with, closely spaced from, and susbstantially parallel to said print face, located between said print face and said record medium,

means to create a disruptive discharge between said pin electrodes and said grid for generation of clouds of ions, and

a biasing source poled to maintain said grid electrode positive relative to said back electrode, thereby creating an electric field.

5. In an electrostatic printing apparatus for establishing electrically charged areas of controllable size in a communicative arrangement on a charge retentive medium, said printing apparatus having a plurality of ionization initiating pin electrodes terminating in a print face, said pin electrodes being energizable by a means capable of selecting one or more of said pin electrodes, said printing apparatus having a back electrode being maintained at a reference potential and spaced apart from and parallel to said print face and serving as a support for the charge retentive medium, the improvement of gaining large more intensely charged areas while maintaining the charge retentive medium at a relatively great distance from said print face without the need to maintain an operating voltage dependent upon the distance between said print face and said back electrode, the improvement comprising:

an electrically conductive grid between said print face and said charge retentive medium spaced parallel to and less than mils from said print face and electrically biased to a potential insufficient to charge said retentive medium, but upon energization of one or more of said pin electrodes ions are introduced into an electric field existing between the grid and the charge retentive medium rendering the field sufficient to form electrically charged areas on the medium.

6. In an electrostatic printing apparatus for establishing electrically charged areas of controllable size in a communicative arrangement on a charge retentive medium, said printing apparatus having a plurality of ionization initiating pin electrodes terminating in a print face, said pin electrodes being energizable by a means capable of selecting one or more of said pin electrodes, said printing apparatus having a back electrode being maintained at a reference potential and spaced apart from and parallel to said print face and serving as a support for the charge retentive medium, the improvement of gaining large more intensely charged areas while maintaining the charge retentive medium at a relative great distance from said print face without the need to maintain an operating voltage dependent upon the distance between said print face and said electrode, the improvement comprising:

a mesh screen of electrically conductive material having a density of substantially 50 to 1,000 lines per inch, said screen being between said print face and 70 said charge retentive medium and spaced parallel to and less than 10 mils from said print face and electrically biased to a potential insufficient to charge said retentive medium, but upon enerigization of one or more of said pin electrodes ions are introduced into an electrical field existing between the screen and the charge retentive medium rendering the field sufficient to form electrically charged areas on the medium.

7. An electrostatic printing device comprising:

a plurality of ionization initiating pin electrodes whose operating ends terminate in a common plane forming a print face,

a planar back electrode extending substantially parallel but spaced from said print face to form a gap into which a charge retentive medium is introduced,

means to energize a selected one or more of said pin electrodes, and

means to establish individual, substantially non-divergent ion fields, each in line with, but substantially larger than the cross-sectional area of the operating end of each such energized pin electrode, each said field extending in non-divergent form for the major portion of the distance between the print face and the back electrode.

8. An electrostatic printing device comprising:

a plurality of ionization initiating pin electrodes whose operating ends terminate in a common plane forming a print face,

a planar back electrode extending substantially parallel but spaced from said print face to form a gap into which a charge retentive medium is introduced,

means to energize a selected one or more of said pin electrodes, and

a mesh screen of electrically conductive material having a density of substantially 50 to 1,000 lines per inch, said screen being between said print face and said charge retentive medium, and spaced parallel to and less than 10 mils from said print face and electrically biased to a potential insufiicient to charge said retentive medium, but upon energization of one or more of said pin electrodes ions are introduced into an electric field existing between the screen and the charge retentive medium rendering the field sufiicient to form electrically charged areas on the medium.

9. An electrostatic printing device comprising:

a plurality of ionization initiating pin electrodes Whose operating ends terminate in a common plane forming a print face,

a planar back electrode extending substantially parallel but spaced from said print face to form a gap into which a charge retentive medium is introduced,

a mesh screen of electrically conductive material having an area substantially coextensive with said print face, said screen having a mesh density of substantially 50 to 1,000 openings per inch, and

means mounting said screen upon the print face in electrical insulating relation to the pin electrodes and in substantially parallel coextensive relation to the print face and within 10 mils thereof.

References Cited UNITED STATES PATENTS 2,817,765 12/1957 Hayford.

2,879,395 3/ 1959 Walku-p 346-74 3,195,142 7/1965 Benn 346-74 3,295,440 1/ 1967 Rarey.

3,358,289 12/1967 Lee 346-74 3,372,400 3/1968 Epstein 34674 BERNARD KONICK, Primary Examiner.

LEE J. SCHOEDER, Assistant Examiner U.S. Cl. X.R.

P0405" UNITED STATES PATENT OFFICE 569 CERTIFICATE OF CORRECTION Patent No, 314601156 Dated August 5, 1969 Theodore E. Byrd Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' Column 4, line 2 "biasted" should read --biased--. Column 4, line 31 "different" should read -differently-. Column 5, line 66 before "electrode" insert -back--. Column 5, line 74 "enerigization" should read energization-.-.

SI'GN'ED 'AND SEALED APR 2 81970 Attest:

Mo FlCtchcrg II. E. 5am, m. Attesting Officer oner of Patents 

